In database.upsert, how can I find the updated or inserted records based on externalID?

I am upserting list of records based on externalId as follows:

List results = Database.upsert(lstRecords, CustomObject__c.External_Id__c,true);
The results returns as follows:

(Database.UpsertResult[getErrors=();
                        getId=a1k0j000000P6CmAAK;
                        isCreated=true;
                        isSuccess=true;], 
 Database.UpsertResult[getErrors=();
                        getId=a1k0j000000P6CNAA0;
                        isCreated=false;
                        isSuccess=true;])
The first record is inserted and the 2nd record is updated.

Since I am upserting based on externalId, I need to know for which ExternalId the records have been successfully inserted or updated.

UpsertResult Class doesn't expose any methods. It returns SalesforceId and NOT externalId

I need a results structure like this:

(Database.UpsertResult[getErrors=();
                        getExternalId = '1234';
                        getId=a1k0j000000P6CmAAK;
                        isCreated=true;
                        isSuccess=true;], 
 Database.UpsertResult[getErrors=();
                        getExternalId = '1235';
                        getId=a1k0j000000P6CNAA0;
                        isCreated=false;
                        isSuccess=true;])
What would be the best way to achieve this?